The oil industry uses core guns to take samples of the side wall of a borehole. The core guns, which are lowered downhole, have plural core cups that are individually fired radially outward into the borehole wall for obtaining samples. Each core cup is fitted with an ignitor and an explosive charge.
One core gun may contain as many as 24 core cups. The firing of each individual core cup is controlled from the surface through the use of a rotary stepping switch located downhole. The stepping switch steps through multiple positions, with each position providing electrical continuity to a respective core cup ignitor. The stepping switch is stepped from one position to the next by a solenoid, also located downhole. When the solenoid is energized by a signal from the surface, the solenoid causes the stepping switch to rotate to the next position wherein a new ignitor can be fired.
Prior art solenoids are subject to several disadvantages when used in a downhole environment. Because the shafts of prior are solenoids accelerate throughout their rotational movement, the shafts tend to over rotate and cause the switch to jump 2 or 3 positions, thereby missing some ignitors. Prior art solenoids overcome this overrotation by compensating with devices that break the shaft momentum and limit the amount of shaft rotation. These limit devices complicate the workings of the solenoid and add to the cost. Furthermore, there is some concern that these prior art solenoids will not operate satisfactorily in a downhole environment because vibration and shock from the detonation of the explosive charges could impair the function of the limit devices.